1. Field of the Invention
The invention provides methods for synthesizing borane compounds comprising between about 5 and about 96 boron atoms or more preferably between 5 and 36 boron atoms. The invention further provides isotopically enriched boron compounds prepared by the aforementioned methods. In certain aspects, the invention relates to B18H22, including 10B- and 11B-enriched B18H22, and methods of preparing same.
2. Background
Large boron hydride compounds have become important feed stocks for boron doped P-type impurity regions in semiconductor manufacture. More particularly, high molecular weight boron hydride compounds, e.g., boron hydride compounds comprising at least a five (5) boron atom cluster, are preferred boron atom feed stocks for boron atom implantation.
An important aspect of modern semiconductor technology is the continuous development of smaller and faster devices. This process is called scaling. Scaling is driven by continuous advances in lithographic process methods, allowing the definition of smaller and smaller features in the semiconductor substrate which contains the integrated circuits. A generally accepted scaling theory has been developed to guide chip manufacturers in the appropriate resize of all aspects of the semiconductor device design at the same time, i.e., at each technology or scaling node. The greatest impact of scaling on ion implantation processes is the scaling of junction depths, which requires increasingly shallow junctions as the device dimensions are decreased. This requirement for increasingly shallow junctions as integrated circuit technology scales translates into the following requirement: ion implantation energies must be reduced with each scaling step. The extremely shallow junctions called for by modern, sub-0.13 micron devices are termed “Ultra-Shallow Junctions” or USJs.
Methods of manufacturing boron doped P-type junctions have been hampered by difficulty in the ion-implantation process using boron. The boron atom, being light (MW=10.8), can penetrate more deeply into a silicon substrate and diffuse throughout the substrate lattice rapidly during annealing or other elevated temperature processes.
Boron clusters or cages, e.g., boranes have been investigated as a feed stock for delivering boron to a semiconductor substrate with reduced penetration. For example, as recited in commonly assigned International Patent Application PCT/US03/20197 filed Jun. 26, 2003, boron ions may be implanted into a substrate by ionizing boron hydride molecules of the formula BnHm (where 100>n>5 and m≦n+8) and an ion source for use in said implantation methods. Certain preferred compounds for use in the boron ion implantation methods included decaborane (B10H14) and octadecaborane (B18H22).
Large boron hydride compounds, that is boron compounds having between 5 and about 100 boron atoms (more typically between 10 and about 100 or between 5 and about 25 boron atoms) are preferred for use in molecular ion implantation methods for delivering boron atoms to a semiconductor substrate. Typically two or more structural isomers exist of large boron hydride compounds, e.g., two or more compounds having the same chemical formula but different structural arrangement of boron atoms in the cage structure. In addition, two or more structurally related boron hydride compounds having the same number of boron atoms but different numbers of hydrogen atoms have been isolated for various sized boron clusters. Such compounds are frequently referred to as closo (BnHn), nido(BnHn+2), arachno (BnHn+4), hypho (BnHn+6), conjuncto (BnHn+8), and the like. Thus, a plurality of different boron hydride species, including structural isomers and compounds containing various amounts of hydrogen are frequently known for boron hydrides having n boron atoms. See, for example, Jemmis, et al., J. Am. Chem. Soc., v. 123, 4313-4323 (2001), which provides a review of various macropolyhedral boranes and known compounds having n boron atoms and various amounts of hydrogen.
Mixtures of structural isomers and mixtures of n-boron atom containing boron hydrides are suitable for use in the implantation methods, in part, because the molecular ions generated by the ionization process of boron hydride mixtures will have uniform and narrow weight distributions.
Current synthetic technologies for the preparation of large boron hydride molecules, e.g., boron hydride molecules with more than 12 boron atoms, are often plagued by complicated synthetic processes, low isolated yields, and/or inconsistent reproducibility.
Kaczmarcyzk (J. Am. Chem. Soc., v. 96, 5953-5954 (1974)) and Graham (U.S. Pat. No. 3,350,324) recite methods of fusing anionic dodecaborane polyhedral cages to generate macropolyhedral clusters. For example, Kaczmarcyzk recites degrading a conjugate acid of the dodecaborane anion in water followed by addition of tetramethylammonium hydroxide to generate a [Me4N]5B48H5 and [Me4N]2B24H23. Graham recites a similar procedure.
For example, several reports issued in the mid-1960's regarding methods of preparing B18H22 by degradation of the conjugate acid of [B20H18]2−. However, each of the synthetic procedures disclosed in these references have not been reproducible or offer the final product in unacceptably low yield.
Olsen described the preparation and exploratory chemistry of B18H22 in a paper published in The Journal of the American Chemical Society (J. Am. Chem. Soc., v. 90, 2946-2952 (1968)). Olsen recites a certain method of preparing B18H22 which involves passage of a salt of the [B20H18]2− anion, dissolved in a mixed solvent of 90% absolute ethanol and 10% acetonitrile, through an acid ion-exchange column to yield a yellow solution of the hydrated conjugate acid of the B20H18 anion, e.g., H2[B20H18].xH2O. The solution is concentrated under vacuum and as the last traces of volatile solvent are removed, the yellow solution undergoes an exothermic reaction evolving appreciable quantities of hydrogen gas. After about 20 minutes, the evolution of gas ceases and a viscous yellow oil results. After an additional 12 hours on the vacuum line, the yellow oil is subjected to an extraction using a mixture of cyclohexane and water. The cyclohexane layer is separated from the water using a separatory funnel. Removal of cyclohexane yields B18H22 as a mixture of two isomers. Olsen reports an isolated yield of 53%.
In a preliminary disclosure, Hawthorne reports a similar procedure to the synthetic protocol of Olsen in which the initial solvent and the extraction conditions are modified (J. Am. Chem. Soc. 87, 1893 (1965)). That is, the residue of the conjugate acid, after concentration, is extracted with ether and subsequent addition of water induced effervescence. Hawthorne reports an isolated yield of 60% of B18H22 after purification by fractional crystallization and sublimation.
The Hawthrone publication further reports certain methods for the preparation of salts comprising the [B20H18]2− anion.
Chamberland recites the preparation of (H3O)2B20H18.3.5H2O by passing a salt of the [B20H18]2− anion through an acidic ion-exchange resin (Inorganic Chemistry, v.3, 1450-1456 (1964)). Chamberland teaches that the conjugate acid of the [B20H18]2− anion is unstable and decomposes slowly to form B18H22 and boric acid. However, Chamberland fails to report a yield or level of conversion for this procedure.
U.S. Pat. No. 6,086,837, issued to Cowen, et al., relates to multi-step methods of synthesizing isotopically enriched decaborane, which methods include extensive purification processes and the use of enriched decaborane in boron neutron capture therapeutic pharmaceuticals.
Although there are several synthetic routes reported in the literature for the preparation of large boranes, they are lengthy and often produce compounds in notably low yields. It thus would be desirable to have new methods to synthesize boron hydride compounds. It would be particularly desirable to have new methods to synthesize BnHm compounds (where n is between 5 and 48 and m≦n+8) and more preferably to synthesize B18H22, B20H24, and related large boron hydride cluster molecules.